1. Field of the Invention
The present invention relates to a process for underfill encapsulating a flip chip driven by pressure, and more particularly to an encapsulation process in which encapsulation of the next chip can begin without waiting the encapsulant packaging the prior chip to cure.
2. Description of the Prior Art
Due to the demand on high-density and high-power electronic packaging, flip chip technology has become important in many fields. The so-called flip chip technology is characterized by flipping over the bare die for attaching to a substrate. When an organic material is used as the substrate, the great difference in the coefficient of thermal expansion between the organic substrate (14-17 ppm/.degree. C. and the silicon wafer (4 ppm/.degree. C.) causes significant strain on the solder connections during temperature cycling, which results in easy deterioration of the connections.
Therefore, to enhance reliability, encapsulant is usually filled into the space between the substrate and the chip. In this way, the stress can be dispersed to the encapsulant so as to decrease the stress taken by the connections. Thus, the connection cracking can be alleviated, the crack will not extend, and the fatigue life of the connections can be prolonged. In addition, the encapsulant can also prevent the transmission of leak current caused by impurities between the solder connections. Statistic data show that the reliability of the chip can be increased five to ten times once underfill encapsulation is conducted. Therefore, underfill encapsulation is a necessary process. However, it suffers from the long time required for underfilling and curing the encapsulant.
Currently, most flip chip packages are encapsulated by dispensing a liquid encapsulant with low viscosity along the periphery of the chip. Capillary action, which is generated from the encapsulant in the fine space (less than 100 .mu.m) between the chip and the substrate, drives the encapsulant to fill the gap between the solder connections. The disadvantages of such a process include: (1) Since the filling is conducted by capillary action, it is very slow. This problem becomes even more serious as the chip size increases because the filling time is proportional to the square of the length of the chip. For example, in a typical encapsulation operation, the filling takes several minutes to several tens of minutes for a 7 mm.sup.2 chip depending on the filling temperature. (2) Since the capillary action is insufficient and the pressure can not be effectively maintained, voids are easily formed in the encapsulant when the filling is complete. Moreover, the adhesion on the interface is also insufficient. Thus, the reliability is adversely affected. (3) For the sake of the environment, liquid encapsulation should be reworkable, which can prevent the problems of known good die (KGD). However, since thermoplastic material is introduced into the epoxy resin (encapsulant), the viscosity of the encapsulant will be greatly increased, which makes the dispensing process more difficult.
Recently, the Cornell Injection Molding Program has provided an underfill encapsulation process in a patent (U.S. Pat. No. 5,817,545, Wang et al.) assigned to Cornell Research Foundation, Inc., which can solve the disadvantages of the above-mentioned dispensing process. The feature of such a process is that the underfill encapsulation process is conducted in a hermetic environment, that is, in a mold. Since the chip is hermetically sealed by the mold, the driving force for filling the encapsulant into the gap need not to be generated from capillary action, but results from the pressure applied to the dispensing device. Thus, the filling time is short and the pressure can be effectively maintained. Such an underfill encapsulation process driven by pressure has the following advantages: (1) Since the filling process is driven by applied pressure, the filling time can be reduced to several seconds. (2) Since the filling time is short, the liquid encapsulant with faster curing kinetics can be used, thus saving the curing time. (3) Since the filling is driven by pressure, the encapsulant material suitable for use in this process can have a 1000 times higher viscosity than that suitable for use in a dispensing process. The selection of encapsulant will become much more flexible. Therefore, encapsulant materials with higher viscosity, for example, those which can be reworkable, with higher adhesion force, and which are filled with non-ball shaped silica, can be used in this process. (4) The reliability of the products can be increased. Since the pressure is increased in this process, voids can be effectively lessened, adhesion in the interface can be increased, and encapsulation can be conducted at room temperature. Also, the periphery of the chip can be packaged into a fillet shape. All those are helpful to increase the reliability.
Having the above advantages, however, the Wang's underfill encapsulation process driven by pressure still suffers from some problems. When the wafer is encapsulated, since the mold is fixed to the encapsulating machine, in order to prevent the encapsulant to stick to the mold, the mold can not be removed until the encapsulant is cured. After the encapsulant is cured, the mold is removed, then, another wafer is placed in the mold for encapsulation. The curing of the encapsulant takes several tens of seconds to several minutes, which makes the entire process long.